1. Field of the Invention:
The present invention relates to a method of and an apparatus for successively transmitting digital data signals using the time-division multiplex access (TDMA) technique and automatically equalizing and demodulating the received digital data signals into output signals based on channel information extracted from the received digital data signals.
2. Description of the Related Art:
As shown in FIG. 1 of the accompanying drawings, data transmission in a TDMA system involves the successive transmission of N-channel data signals in a frame format 32 that are allocated respectively to N time slots 30, 31, each composed of N symbols. The first symbol Np of the frame in each of the time slots 30, 31 comprises a training sequence.
In order to eliminate intersymbol interference caused during data transmission as well as to cope with time-dependent variations in channel characteristics, data receivers in such a TDMA system extract channel information using the training sequences in the received time slots for initializing equalizers and demodulate the received data signal in each of the time slots.
As communication channels are designed for faster, multiplexed communications, short-term variations in the channels become more problematic. There have been proposed various systems capable of quickly following or compensating for burst variations which cannot be ignored in received time slots.
One of the proposed systems is shown in FIG. 2 of the accompanying drawings. The system includes an input terminal 70, an output terminal 71, a channel estimator 72, and a demodulator 73. The channel estimator 72 estimates channel information using a received signal from the input terminal 70 and a decision result from the demodulator 73 to compensate for channel variations (see, for example, K. Okanoue, Improvements on Tracking Performances of Adaptive Viterbi MLSE Receiver, trans. of 1991 Spring National Conference, Institute of Electronics, Information and Communication Engineers, Tokyo, SB-4-4, pp. 2-624, 625, March 1991).
Another proposed system, which is shown in FIG. 3 of the accompanying drawings, has an input terminal 80, an output terminal 81, a channel estimator 82, and a demodulator 83. The channel estimator 82 establishes an equalizer using only received signals from the input terminal 80 and not a priori channel information to compensate for channel variations (see, for example, A. Ushirokawa et al., Viterbi Equalization on Time-varying Channel, 2nd Makuhari Int. Conf. on High Tech., Chiba, A-2-2, pp. 101-104, January 1991).
FIGS. 4 and 5 of the accompanying drawings show still another proposed system. As shown in FIG. 4, the system employs a signal format including at least two training sequences 91, 92 in one time slot 90 and interpolates a plurality of results estimated from the training sequence signals into channel information. An equalizer is established using the channel information thus created to compensate for channel variations. As shown in FIG. 5, the system includes a memory 1002 for storing a signal received in one time slot 90 from an input terminal 1000. Channel estimators 1003, 1004 are supplied with respective training sequence signals 91, 92 from the memory 1002, and estimate channel information at the times the respective training sequence signals 91, 92 are received. The system also has a channel information generator 1005 for combining the estimated channel information from the channel estimators 1003, 1004 into channel information over all the times in the time slot 90. The channel information generator 1005 outputs the produced channel information to a demodulator 1006. The demodulator 1006 demodulates the signal in the time slot 90 stored in the memory 1002, based on the channel information supplied from the channel information generator 1005. The demodulated signal is sent to an output terminal 1001. For more details, see, for example, S. Sampei, Complexity Reduction of RSL-Decision Feedback Equalizer using Interpolation, trans. of 1991 Spring National Conference, Institute of Electronics, Information and Communication Engineers, Tokyo, B-386, pp. 2-386, 1991.
Unfortunately, the above proposed systems suffer the following drawbacks:
The system shown in FIG. 2 controls the demodulator 73 using the decision result from the demodulator 73. If the decision result contains an error, the system will fail to compensate for channel variations, and the error will be transmitted, greatly degrading the characteristics of the received signal. The system shown in FIG. 3 does not produce such an error and is capable of effectively compensating for channel variations. However, because its algorithm for establishing an equalizer to compensate for channel variations is complex, the system is relatively large in scale. The system shown in FIGS. 4 and 5 is not large in scale and has a high capability to compensate for channel variations. However, it requires a plurality of training sequences in one time slot, resulting in a reduction in transmission efficiency. As the speed of channel variations increases, the system fails to compensate for the channel variations with sufficient accuracy based only on estimated values from the conventional training sequences and requires more training sequences, which in turn further reduces transmission efficiency.